Thancred and Y'shtola's losses both work as character development tools and to establish that the work the WoL and the Scions do requires sacrifice. Its not easy and it has a cost. Its a testament to their belief and commitment to their cause that they take up those wounds and carry on regardless.

I still think at some point we will see Matoya sacrifice something to aid or save Y'shtola, probably when her apprentice has pushed her limits too far. The side quests show how important Y'shtola is to Matoya.